The rate of dissociation of calmodulin from cyclic nucleotide phosphodiesterase is increased as the level of calcium is decreased, indicating complex formation between the enzyme and calmodulin with less than four calcium ions bound. ACTH was found to inhibit the activation of phosphodiesterase by calmodulin by forming a complex with the regulatory protein. Preliminary experiments showed that yeast contains neither calmodulin nor phosphodiesterase activatable by bovine brain calmodulin. (2) The mechanism of activation of proteinase B in the chitin synthetase cascade in yeast was studied. The slow process seems to reflect a slow conformational rearrangement. Activation of proteinase B by SDS was found to be due to increased solubility of the substrate azocoll by the detergent, not due to dissociation of proteinase B inhibitor. (3) With 2-amino-4-phosphorobutyric acid as a deadend competitive inhibitor for glutamine, the kinetic mechanism of the gamma-glutamyltransferase reaction of E. coli glutamine synthetase can be shown to be rapid-equilibrium-random. (4) An extension of the theory of continuous variation for the determination of binding stoichiometry is presented. It provides a guide for using the relative magnitudes of protein-ligand complexes at different points to determine the reliability of the data and to calculate the dissociation constant.